Direct-current amplifier



2 sneefs-sheet 1 LAL.,

R. T. CAVANAGH DIRECT CURRENT AMPLIFIER July Z7, 1948.

Filed May s, 1947 `Iuly 27, 1948. R. T. cAvANAGH 2,445,938

' DIRECT CURRENT AMPLIFIER v Filed May s, 1947 2 sheets-sheet 2 Patented July 27,1 1948 Robert Terrance Cav anagh, Toronto, Ontario,

Canada, assignor to Allen B. Du Mont Laboratories, Delaware Inc., Passaic,` N. J., a corporation of` Application May 8, 1947, Serial No. 746,750

. 1 'Ilhis inventionrelates to an amplifier that is capable of providing voltage gains up to about 106 with very little if any distortion in a range of frequencies from directcurrent Ito about 50 kc.

It is particularly suitable for amplifying signals i of amplitudes less than 1000 microvolts.

With this invention a high frequency carrier is modulated to a high degree by a low level signal of frequencies from to 50 kc. or above. Provision is made for tuning the circuits in accordance with the frequencies that are to be used. The percentage of modulation of the carrier can be controlled, and a low level carrier output voltage is made available so that high gain intermediate frequency amplincaticn of the modulation envelope andwcarrier can be obtained without overloading and consequent distortion.

The invention may be understood from the description in connectio with .the accompanying drawing, in which Fig. 1 is a block invention, and

Fig. 2 is a circuit diagram showing details of the invention.

Fig. 3 is a circuit diagram of another embodiment of the invention; f i

i 'Inthe drawing, Fig. 2, reference characters I and 2 indicate pentodes having their cathodes and suppressor grids grounded. Ba'sing battery 3 is provided to bias the control grids of the tubes diagram of connections for the land 2 negatively. A battery 4 is provided .to I

rsupply current to the plates of the tubes I and 2 through resistances 5, 5', and variable induct- `ances 6, 6 of the same sizes are in series with saidresistances. A resistance 'l is provided between the screen grids of the tubes I and 2 with a sliding Contact 8 thereon that is `connected lto an intermediate point on the battery 4.

Triodes 9 and I 0 have `their plates connected to a source of positive potential such `as the battery I I through lead I 2. The control gridsI-S and I4-of the tubes 9 and Ill are connected through `resistances I and I6 respectively to the plates of the tubes Iland2. i

. Pentodes I1 and I8 'have their screen grids connected respectively to the icathodes of :the tubes 9 `and IIl. The rcathodes and suppressor grids of the ,tubes I1 and I8 are grounded. The control grids oftubes I'l 4and I8 are connected to one end of 'the secondary 20 of a transformer T, to the primary 2I of which acarrier current e2 is applied. 'Iheplates of tubes I'l 4and I8 are connected to opposite ends of the primary 22 of a transformer T that has a source 23 of positive potentialconnected to a. point half way between 12 Claims. (Cl. 179-1715) the ends of this primary. A tuning condenser 2d is also connected between the ends ofthe primary 22. Either :condenser 24 or the inductance of primary 22 may be varied to achieve resonance with the carrier frequency e2. The ends of this primary are connected to the plates 32 and 31 of 4tubes Il and I8, respectively.

The secondary 25 of this transformer T has i a `tuning condenser 26 connected across it and leads are provided at the ends of this secondary for the output eo. Either condenser 26 or the linductan'ce o-f secondary 25 may be varied to achieve resonance with the carrier e2.

Condenser 24 and inductance 22 and condenser 2t and inductance 25 are tuned so .that the transformer` T has a band pass characteristic such that a at response is obtained for 100 kc. on each side of the @carrier frequency ez `for a specified unbalance voltage on the screens of 4tubes I and 2.

The operation is as follows:

A direct or alternating current signal e1 of small i amplitude, which is to be amplified and used to modulate a high frequency carrier, is appliedito the control grids of the push-pull ampliiier i, 2. If the contact 8 that is connected to battery il is not at the center of the resistance 'I that is connected between the screen grids of tubes l and 2, the circuit is unbalanced so thatthe out- -Duts from the plates of tubes I and 2 differ thus causing a difference voltage, which may be desighated es to be developed between grid I3 of tube "9 and grid I4 of tube Ill. The magnitude of this voltage depends upon .the amount of unbalance of resistance 'I and/or the Vmagnitude of the input voltage e1. y t

The voltage c2, which is `that of the carrier frequency, is at the same time applied to the primary 2| of transformer T thus varying Athe plate currents of -tubes I1 and I8 inparallel class B operation, the grid bias for this class of operation being supplied by battery 2l.`

Since the outputs of the tubes I and 2 are applied to the grids of tubes 9` and I0 and the outputs from the cathodes of these `.tubes 9 and IE! Aare applied to the screen grids 35i and 35 of tubes I1 and I8, the transconductances of tubes Il and I8 are varied 180 out of phase in accordance with the input signal e1. The secondary 2li applies the carrier frequency ez to the control grids 29 and 34 of tubes I'I and I8 at the saine time. By virtue of the fact that the cathode of tube 9 is connected to the screen grid 3B of tube I'l, and the cathode of -tube Ill is connected to the screen `grid 35 of tube I8, -the voltage betweenuthe screen grid 30 of tube I1 and the Iscreen grid 35 of tube I8 varies in accordance with the input voltage e1.

A brief analysis of the operation is as follows:

Let the voltage from grid 29 to cathode 2B of tube I1 be egi, the voltage from grid 34 to cathode 33 of tube I8 be ege, the voltage from screen grid 3U of tube Il to screen grid 35 of tube i8 be e4, the voltage from screen grid 30 to cathode 28 be e5, and the voltage from screen grid 35 to cathode 33 be ec.

Then eli--es-l-es Let the carrier voltage appearing in the grid circuit be e7, and the battery 21 voltage be E.

e7=C (cos wit) where C is a constant, w7=w2, where wz is the angular frequency of the carrier Voltage e2.

but cm1 is a function of e7 and egi, or cm1: fleur, e7)=a1+bie+c1e2+d1es3+ where es=As(1-{-cos Wst) +B1(E+cos wrt) The frequency of the input voltage e1 is L w1 f5`21rh27r where w1=the angular frequency of input voltage e1.

A5 is a constant depending on the voltage from screen grid 30 to cathode 2S of tube l?, and the relative effect of this screen voltage on the transconductance ami. B1 is a -constant depending on the relative effect of grid voltage cgi on the transconductance cm1.

Ae and B2 are the same to gmz as A5 and B1 to gmi above and where w1 is as specified above.

If the gm series are assumed to converge, a finite value ofy plate current is obtained for each tube. This plate current contains all the usual modulation components, those of importance being of frequencies wz, (wz-w1) and (wz-Hm).

The output voltage eo depends on the difference plate current components p1-ipa Thus with slight static off-balance of the input volttage e4 to the modulator by means of tap 8 on resistor 1, the output voltage eo contains c-omponents of frequencies wz, (wz-w1), (wz-l-wi).

The transformer T is tuned for a band pass characteristic of 200 kc. so that only the carrier frequency and usual sideband frequencies appear as components of the output voltage eo from T. The output Voltage en is a conventional amplitude modulated wave of carrier frequency e2, which may for example be 4.0 mc., the modulation envelope being determined by the amplitude and frequency of ei.

The output from the secondary 25 is fed to the first intermediate frequency amplifier indicated in Fig. 1. The remaining portion of Fig. 1 indicates the conventional D. C. means for detecting and observing a signal that has been transmitted by a carrier.

In Fig. 2, batteries are shown as supplying the potentials necessary for operation. The same results may be achieved by using conventional power supplies and self biasing arrangements employing cathode resistors in the well-known manner.

Fig. 3 is a diagram showing another embodiment of the invention. In this embodiment the screen currents of tubes I1' and I8' are supplied through the plate loads 38, 39 and 40, 4| of tubesl* and 2,', respectively. This eliminates the cathodefollower tubes 9 and l0 of Fig. 2. The operation is similar to that of Fig. 2.

What is claimed is:

1. An amplifier for low amplitude signals which comprises push-pull pentodes, one of which has its screen grid biased to a higher potential than the other, cathode follower tubes to which the outputs of said pentodes are respectively coupled, a modulator to which the cathodes of said follower tubes are coupled and means to apply a carrier signal to said modulator.

2. The amplifier of claim 1 in which said cathode follower tubes are triodes having their grids coupled to a source of direct current.

3. The amplifier of claim 1 in which the cathodes of said follower tubes areconnectedfto the screen grids of said modulator.

Ll. The amplifier of claim 1 in which said carrier signal is coupled to the control grids of said modulator.

5. The amplifier of claim l in which said pushpull pentodes are unbalanced. y

6. The amplifier of claim l in whichmeans are provided to apply an input signal to the control grids of said push-pull `pentodes.

'7. An amplifier for low amplitude signals by means of which a radio` frequency carrier is modulated to a high degree, which comprises a pair of pentode vacuum tubes connected in push-pull with a frequency response between 0 and about 50 kilocycles 'per second, said amplifier being unbalanced by means of controlled transconductances of said tubes, separate cathode follower stages driven by said amplifiers, two additional pentode vacuum tubes having their screen grids connected to the outputs of said cathode follower stages, said last mentioned tubes havingV impressed upon their control grids a voltage of carrier frequency, a tuned transformed beingprovided as the push-pull plate load of said last mentioned pentodes, whereby an outputA voltage is produced comprising said carrier frequency voltage and side-band frequency voltage` components thereof. I

8. The amplifier of claim 7 in which said cathode follower stages are triodes. .v

9. An amplifier for low level signals by means of which a radio frequency carrier is modulated to a high degree which comprises a pair of'pentode vacuum tubes connected in push-pultwith a frequency response betweenr 0 and about 50 kilocycles per second, said amplifier being. unbalanced by means of controlled transconductances of said tubes, two additional pentode vacuum tubes having their screen grids connected respectively to the plates of said last mentioned tubes, the control grids of said last mentioned tubes being connected in parallel andv having impressed thereon a voltage ofA carrier frequency, atuned transformer being provided as the push-pull plate load of said last mentioned pentodes, whereby an output voltage is produced comprising said carrier frequency voltage and side-band frequency voltage components thereof. Y

10. A device for amplitude modulating a hig frequency carrier wave which comprises pushpull pentodes having their control grids connected to a source of carrier voltage and their carrier frequency connecting the plates of said pentodes.

12. A device for amplitude modulating a high frequency carrier wave which comprises pushpull pentodes having their control grids connected to a source of carrier voltage and their screen grids having unbalanced symmetrical input signals connected thereto from a push-pull amplifier in which the screen grids are unbalanced as to the potential at which they are maintained, and a resonant circuit tuned to said carrier frequency connecting the plates of said pentodes.

ROBERT TERRANCE CAVANAGH.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Tubbs 1 Oct. 16, 1934 Laport Apr. 13, 1937 Gillespie Apr. 4, 1944 Number 

